(1) Field of the Invention
The present invention relates to a video decoder according to MPEG-2, 11/1993 (Moving Picture Experts Group) standards. More specifically, it relates to a video decoder which performs the step of run-length decoding just before the step of inverse transformation so that the number of pipelines can be reduced and also the number of buffers can be reduced.
(2) Description of the Prior art
The growth of multimedia is based on the image compression techniques and progress in digitization. New techniques for digitizing data has been developing rapidly. Accordingly, image compression techniques, which are the basis for the growth of multimedia, are becoming more and more important.
MPEG, which was born in such a technical environment, is the international standards for the compression and encodement of a digital dynamic image, MPEG is an essential technique in the multimedia environment.
MPEG standardization is related to MPEG Video, MPEG Audio and MPEG systems. MPEG Video standardizes algorithms for compressing a video signal. MPEG Audio standardizes algorithms for compressing an audio signal. MPEG system addresses problems of synchronizing and multiplexing the compressed video and audio bit streams for multi-stages.
A function of MPEG is to remove redundancy between frames. MPEG adds to the frame compression functions of JPEG (Joint Photograph Experts Group), standards for compressing static images.
Before describing a video decoder that performs to MPEG standards, the process of encoding an MPEG video signal will is explained.
The process of encoding an MPEG video signal comprises the steps of preprocessing, transformation, quantization, and variable length coding.
In the step of preprocessing, digital image data is sampled or filtered causing a screen to have a prescribed size.
The step of transformation draws relationship between data. Waveform coding such as adaptive differential pulse code modulation (ADPCM), and transformation coding using discrete cosine transformation (DCT) are widely used in the transformation step.
If the discrete cosine transformation method is used, a phenomenon appears in which correlation between the calculated coefficients is little and energy is concentrated into low frequencies. In this case, if processing is performed in the subsequent quantization step, an image of high quality can be obtained with high compressibility.
In the step of quantization, transformed data is represented by a restricted number of bits to reduce data size.
In the step of variable length coding, the data values which, statistically, appear frequently are represented by a small number of bits, data values which appear rarely are represented by a large number of bits. Thus, the total number of bits is reduced.
Bit streams, which are obtained in the above step of variable length coding, are packeted, then recorded in recording media such as a compact disk (CD) or a hard disk, or transmitted through a cable transmission channel or a radio communication channel.
Bit streams, which are recorded or transmitted as above, are reproduced into a video signal by an MPEG video decoder.
In the following, a prior art MPEG video decoder will be described in conjunction with the accompanying drawing. FIG. 2 is a block diagram showing a conventional MPEG video decoder. As shown in FIG. 2, a prior art MPEG video decoder comprises: a syntax parser 210, a variable length decoder 212, run-length decoder 214 and a parameter decoder 216, all of which perform the step of parsing. A block buffer 220 interfaces the steps of parsing and of inverse quantization; an inverse quantizer 230 and an inverse scanner 232, all of which perform the step of inverse quantization. A block buffer 240 interfaces the steps of inverse quantization and of inverse transformation. An inverse discrete cosine transformer 250 which performs the step of inverse transformation. A macro block buffer 260 interfaces the steps of inverse transformation and of compensation. A motion compensator 270 performs the step of compensation.
As described above, a prior art MPEG video decoder has four pipeline stages including the steps of parsing, inverse quantization, inverse transformation, and compensation.
An MPEG video decoder which has the above structure performs decoding in the following order:
Syntax parser 210.fwdarw.Variable length decoder 212.fwdarw.Run-length decoder 214.fwdarw.Block buffer 220.fwdarw.Inverse quantizer 230.fwdarw.Inverse scanner 232.fwdarw.Block buffer 240.fwdarw.Inverse discrete cosine transformer 250.fwdarw.Macro block buffer 260.fwdarw.Motion compensator 270.
If decoding is performed in such order, three buffers are required, and the total number of pipeline stages will be four. If the number of pipeline stages is larger, control will be more complex and a larger number of buffers are required.